The present invention relates to a warm-up promotion device for automatic transmission of the type having a torque convertor, an inlet passage connecting between the oil inlet of said torque convertor and the outlet of a hydraulic pump driven by an engine, and an outlet passage connecting between the oil outlet of the torque convertor and an oil tank, the outlet passage being provided therein with an oil cooler.
In a vehicle mounting an automatic transmission of the type described, a considerable heat is produced in the torque convertor due to a slip in the latter, so that the oil discharged from the torque convertor is cooled by an oil cooler and returned to an oil tank directly or after passing through essential lubricating parts of the automatic transmission. This system operates without any problem under normal condition of use, but still suffers from the following disadvantages to be overcome in the operation immediately after start up of the engine in cold district.
Namely, the oil temperature is not raised sufficiently within a period of several minutes after the start up of engine so that the oil exhibits a large viscosity to increase the resistance in sliding parts, resulting in a power loss and a consequent increase of the fuel consumption. This applies also to rubber parts of oil seals used for keeping oiltight seals on every rotating parts. Namely, the rubber parts of the oil seals in cold state increases the sliding resistance.
In cold districts, it is often experienced that this unfavourable state continues for more than ten minutes. It is a current tendency that the torque convertors of automatic transmissions are used in a "direct connection" condition to reduce the fuel consumption. In such type of automatic transmission, the heat output from the torque convertor is so small that the above-described problem is serious and, therefore, there is a demand for a suitable measure for promoting the warming up of the automatic transmission.
The mechanism for direct connection of the torque convertor is broadly sorted into two types: namely, a first type in which the control is made by making use of the internal pressure of the torque convertor, and a second type in which the control is made by the difference of pressure between the internal pressure of the torque convertor and a separately supplied hydraulic pressure. In either type, the internal pressure of the torque convertor is an important factor for determining the connecting power of the direct connection mechanism. From this point of view, it is necessary that the internal pressure of the torque convertor should be made as free as possible from the influences of other factors. In the conventional automatic transmissions, particularly in those used in a cold district, however, the resistance against the flow of oil is increased as the ambient temperature comes down in the oil cooler located downstream of the torque convertor and in other pipes or conduits associated with lubricating parts. In consequence, the internal pressure of the torque convertor is increased to impair the accomplishment of the initial design of the automatic transmission.